Downforce > Drag

I have two Miatas, and even my faster one is slow. Adding an airdam, splitter, side skirts, wing, and all the other aero bits add weight, and some of them increase drag as well. The last thing you want to do to an underpowered car is add more weight and drag, right? Maybe.

Drag reduction matters most when accelerating on a straight, but pretty much everywhere else downforce is preferable to drag reduction. Even still, there are times when drag reduction is more important, such as in an endurance racing strategy where you want to do one less pit stop. It also seems logical that at a high-speed track you’d want to skew your aero package towards top speed and reduced drag, especially in an underpowered car.

Or so you’d think. But like most things aerodynamic, what seems obvious could be completely wrong. So let’s examine downforce vs drag on a very fast racetrack that is dominated by long straights and top speed.

I did a motorcycle track day at Portland International Raceway, and I’ll describe it like this: it has a really long and boring front straight, a couple corners, another long and slightly less boring back straight, and a couple corners. If ever there was a track where you’d want to reduce downforce and optimize for less drag, this is probably it.

In Race Car Aerodynamics, the author Joseph Katz calculated lap times for a generic prototype race car at Portland International Raceway factoring in grip and drag. Take a look at the track layout in the chart inset, it’s like I said. The full results are in SAE Paper 920349, but this is what I make of it.

At this track, you might think that you should set your aero for the least possible drag, thereby attaining the highest top speed. But that actually sets the worst lap time, some 6 seconds off the pace. Or you might think to optimize for the highest L/D ratio, and with that you’d at least be within the same second as the fastest cars.

But somewhere in the neighborhood of maximum downforce, that’s where the fastest lap time was. On any other track I’d guess that maximizing downforce is the right thing to do, but I’ve raced down the front straight on this track (which is nearly a mile long), and the results are surprising.

This is a calculation, albeit a very sophisticated one, and it’s based on a race car with a lot of power that can overcome drag. Still, it makes me wonder if we should chase all the downforce we can and not worry about drag reduction at all. Miatas are all about cornering anyway, and we’re used to getting passed on the straights!

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